aggregate glands are found in the Argiopidce, Linyphiidœ, and 

 Theridiidœ ; the lobed glands, only in the Thcridiidœ ; and the 

 cribehum glands in the Cribellat.e. Each of these three 

 groups of spiders that possess a characteristic type of silk gland 

 possesses also the first four types mentioned above ; hence 

 the presence of five types of silk glands in a single species is 

 common. 



The form and structure of each of these types of silk glands 

 have been described by Apstein ; for the purposes of this 

 paper it is only necessary to call attention to the fact that the 

 aciniform and pyriform glands are small and very numerous, 

 while the other types of glands (except the cribellum glands) 

 are much larger and quite limited in number. 



Correlated with the differentiation of the silk glands into 

 several types, there have been developed several different types 

 of spinning organs. Each gland opens through a spinning tube ; 

 but the different sets of glands open through different kinds of 

 tubes. Omitting the characteristic details of form distinguishing 

 the different types of spinning tubes, they can be grouped into 

 two kinds : first, the small spinning tubes through which the 

 numerous aciniform and pyriform glands open ; and second, 

 the much larger spinning tubes, termed spigots, through which 

 the larger and much less numerous glands open. The exceedingly 

 small pores through which the cribellum glands open might 

 be considered as a third type. 



In addition to these specialized outlets of the silk glands, 

 we find that in certain families of spiders there is an organ for 

 manipulating the silk after it leaves the spinning tubes ; this is 

 the comb on the hind tarsus in the Theridiidce and the calamis- 

 trum on the metatarsus of the hind legs in the Cribellat^. 



Before entering upon the account of the different kinds of 

 silk spun by spiders, it is necessary to clear the way by reference 

 to a wide-spread error concerning the structure of the most 

 commonly observed threads. 



The fact that the spinnerets bear many spinning tubes which 

 serve as outlets of the numerous spinning glands, led the early 

 writers to conclude that the ordinarily observed threads were 

 each composed of many minute threads united. This view was 

 held till 1890, when Warburton pointed out that the dragline 



